DC-to-AC power converters are commonly used to produce AC power from a DC source, such as a battery. Such DC-to-AC power converters are typically found on boats or utility trucks, where they can be used to power tools, microwave ovens, computers or any other device that requires AC power to operate.
In order to extend battery life, many DC-to-AC converters are designed to reduce the power dissipated within the converter itself when no load is present. One prior scheme to extend battery life has been to place a pair of back-to-back diodes in series with the an output lead of the converter. A resistor is connected between the battery and the diodes to bias the diodes to a minimum voltage (approximately 1.2 volts). Because the output impedance of the converter is low, the voltage provided by the diodes appears across the output leads of the converter. As a load draws current from the converter, a load sense circuit monitors a change in voltage across the resistor to detect the presence of a load and subsequently turns on the DC-to-AC converter. A problem with this approach is that the diodes do not provide a sufficient voltage to start some loads, such as fluorescent lights or electric equipment, and therefore the load sense circuitry is never triggered. Such a power converter has to be turned on manually in order to run the types of loads.
A second prior art method of saving battery power is to periodically turn on the DC-to-AC converter at full power for a brief period of time in order to detect the presence of a load. Each time the converter is turned on, a current monitoring circuit determines if a load is drawing current from the converter. If so, the converter is turned on to provide a continuous output signal. This method overcomes the problem of having an insufficient voltage to start certain loads. However, it has other problems. In order to save battery power, the number of AC cycles for which the power converter operates must be limited during each test pulse. This means that a user may experience some delay when turning on a load because the load may be plugged in between two test pulses. A second problem with this scheme is that some loads will not start until they are driven by an AC voltage that is present for a considerable number of continuous AC cycles. A user wishing to run such a load may again be required to turn on the power converter manually. Therefore, there is a need for a DC-to-AC converter that can start any load while still conserving battery power when no load is present. The present invention is directed to providing such a DC-to-AC power converter.